A temperature controlled analysis (or a temperature-program-controlled analysis) is often used to reduce the duration of an analysis of a gas chromatograph. In the temperature controlled analysis, the temperature of the column of the chromatograph is gradually raised, whereby the speed of the gas flowing through the column decreases because of increase in the flow resistance of the column, though there is a minor opposing factor that the volume of the gas increases as the temperature rises. Therefore, the pressure of the gas is increased as the temperature is raised in order to compensate for the decrease in the speed of the gas.
There is an index called Height Equivalent to a Theoretical Plate (HETP) which represents the resolution of a column of a chromatograph. The value H of HETP is defined as EQU H=L/N,
where L is the length of a column and N is the theoretical number of plates of the column, and is calculated from a chemical kinematic theory as EQU H=B/v+(C.sub.G +C.sub.L).multidot.v, EQU B=2.multidot.D.sub.G .multidot.(1/j), EQU C.sub.G =[(1+6.multidot.k+11.multidot.k.sup.2)/{2.sup.4 .multidot.(1+k).sup.2 }].multidot.(r.sup.2 /D.sub.G).multidot.(1/j) EQU C.sub.L =2.multidot.k/{3.multidot.(1+k).sup.2 }.multidot.(d.sub.f.sup.2 /D.sub.L) EQU j=(3/2).multidot.(P.sup.2 -1)/(P.sup.3 -1) EQU P=Pi/Po EQU k=(C.sub.S .multidot.V.sub.S)/(C.sub.M .multidot.V.sub.M)
where D.sub.G is the diffusion coefficient of the sample in the gas phase. D.sub.L is the diffusion coefficient of the sample in the liquid static phase, r is the inner radius of the column, d.sub.f is the thickness of the liquid static phase layer, k is the capacity ratio, C.sub.S and C.sub.M are the concentrations of the sample in the static phase and in the mobile phase, V.sub.S and V.sub.M are the volumes of the sample in the static phase and in the mobile phase, and Pi and Po are pressures of the carrier gas at the entrance and at the exit of the column. According to the above equations, the value H of HETP changes as shown in FIG. 3 with respect to the linear velocity v of the carrier gas flowing through the column, and there is the minimum in the HETP value at a certain linear velocity v.sub.m where the resolution is the highest. When P is approximately 1, the linear velocity v.sub.m at which the HETP value is minimum is approximated as EQU v.sub.m ={B/(C.sub.G +C.sub.L)}.sup.1/2.
A problem in the conventional gas chromatograph is that, in controlling the pressure of the gas to compensate for the decrease in the gas flow due to the temperature rise, the linear velocity of the gas is not regarded but the rate of the mass flow of the gas is aimed to be constant. Therefore the resolution is not always highest in the conventional gas chromatograph though the analyzing time is reduced.